1. Field of the Invention
The present invention relates to an image reader that reads, by scanning a document placed on a document table, image data from the document and further relates to a corresponding method for adjusting the magnification accuracy.
2. Description of the Related Art
Image readers, such as facsimile machines, image scanners, and copying machines, are configured to read, by scanning a document placed on a document table, image data from the document.
Japanese Unexamined Patent Application Publication No. 9-23318 sets forth an image reader in which a document placed on a document table glass (hereinafter called a document table) reflects a beam emitted from a light source. Such a reflected beam reaches an imaging lens via a series of reflective mirrors. Then, the reflected beam, having reached the imaging lens, is imaged on the imaging surface of a charge-coupled device (CCD). As much as a line of a document can be obtained as image data through this processing.
The light source, reflective mirrors, imaging lens, and the CCD 106 are embedded in an optical unit. The optical unit is placed on two scanning rails. The scanning rails are disposed in parallel with the document mounting surface of the document table and in the sub-scanning direction of a document. The optical unit moves on the scanning rails using a drive unit as a drive source. A motor is embedded in the drive unit.
In such an image reader, the optical unit repeats the aforementioned processing by moving on the scanning rails to obtain the image data corresponding to a page of a document. In this type of image reader, the image data may deteriorate due to the mechanical precision of a read scanning system that includes the optical unit, scanning rails, drive unit, and the like. Phenomena that cause a deterioration of the image data due to the mechanical precision of the read scanning system will next be described.
It is assumed that the optical unit moves on the scanning rails at constant speed. However, the movement speed of the optical unit may partially vary due to, for example, the presence of deformations such as projections and depressions in the scanning rails or a fluctuation of motor rotational speed.
When the movement speed of the optical unit varies while the optical unit is moving on the scanning rails, an error of distance in the sub-scanning direction with respect to a document occurs in the image data obtained during the period. That is, the magnification accuracy in the sub-scanning direction with respect to a document decreases.
Another assumption is that the reflective mirrors in the optical unit have mirror surfaces that are even. However, a curvature may occur in the mirror surface of any of the reflective mirrors. When any of the reflective mirrors has a portion where a curvature has occurred, a reflected beam is scattered at the curvature. As a result, an error of distance in the main scanning direction with respect to a document occurs in the image data, since a portion of the reflected beam is scattered. That is, the magnification accuracy in the main scanning direction with respect to a document decreases.
Typically, the mechanical precision of the optical unit has been adjusted during manufacture or shipment of an image reader, so as to prevent the occurrence of such deterioration of the image data. This has included performing operations such as repairing the deformation of the scanning rails, embedding a high-performance motor having a constant rotational speed, and evening the mirror surface of each of the reflective mirrors repairing the curvature of the mirror surface.
However, such operations increase the manufacturing costs for manufacturing image readers, due to additional labor and expensive parts.